Distinguishing s^± and s⁺⁺ electron pairing symmetries by neutron spin resonance in superconducting NaFe _0.935Co_0.045As

A determination of the superconducting (SC) electron pairing symmetry forms the basis for establishing a microscopic mechanism for superconductivity. For iron pnictide superconductors, the s^±-pairing symmetry theory predicts the presence of a sharp neutron spin resonance at an energy below the sum of hole and electron SC gap energies (E≤2Δ) below T_c. On the other hand, the s⁺⁺-pairing symmetry expects a broad spin excitation enhancement at an energy above 2Δ below T_c. Although the resonance has been observed in iron pnictide superconductors at an energy below 2Δ consistent with the s^±-pairing symmetry, the mode has also been interpreted as arising from the s^{+ +}-pairing symmetry with E≥2Δ due to its broad energy width and the large uncertainty in determining the SC gaps. Here we use inelastic neutron scattering to reveal a sharp resonance at E=7 meV in SC NaFe _0.935Co_0.045As (T_c=18 K). On warming towards T_c, the mode energy hardly softens while its energy width increases rapidly. By comparing with calculated spin-excitation spectra within the s ^± and s⁺⁺-pairing symmetries, we conclude that the ground-state resonance in NaFe_0.935Co _0.045As is only consistent with the s^± pairing, and is inconsistent with the s⁺⁺-pairing symmetry.